The present disclosure relates to aspects of data storage drives, and relates in particular to thermal management in solid-state drives.
Various memory-based data storage drives, such as solid-state drives (SSDs), are formed of a multitude of integrated circuits. Typically an SSD includes one or more printed circuit boards (PCBs), each with solid-state non-volatile-type and/or volatile-type memory or storage components located thereon. A common example of non-volatile memory is NAND memory, and examples of volatile memory include random-access memory (RAM) variations. Solid-state non-volatile memory components for use in an SSD are typically “persistent,” meaning they do not lose data when power is cut off to the memory components. An example SSD may include two PCBs, stacked and spaced vertically, with each PCB containing various solid-state components, and the PCBs may together be located or mounted according to various layouts inside a single drive housing or enclosure.
An SSD typically lacks moving parts (in contrast to a hard-disk drive, which contains at least one spinning magnetic disk), but an SSD may face numerous challenges as the drives have progressively become smaller and denser, while storing more data and becoming more complex. In compact and/or advanced applications where space it at a premium, such as with mobile devices, there may be a greater need for drives that have a smaller outline or form factor, and are both heat-, and power-efficient. Additionally, many SSDs may be located in close proximity to one another, which may compound or exacerbate various heat-related problems, such as overheating. More generally, an SSD may be mounted in close proximity to other heat-producing or variously thermally-sensitive computing components, such as processors and memory. Other sources of heat, such as batteries, may also be present. So, as a result, SSDs typically produce and are subject to undesirable heat that can at times negatively affect performance and longevity. Therefore, the management and dissipation of heat related to an SSD, especially heat created during operation, is an important problem to be solved.
Thermal energy, the transfer or emission of which is called heat, is typically measured in calories, Joules, or British Thermal Units (BTU). Heat is typically measured in degrees of the Celsius, Kelvin, or Fahrenheit scales, as known. Thermal energy is the amount of energy the constituent atoms or molecules of a particular substance that is present due to internal molecular motion. The transfer or communication of thermal energy in the form of heat can take place variously through conduction, convection, and/or radiation. The terms communication and transfer, with regard to thermal energy, may be used interchangeably, herein. Communication or transfer may refer to conduction, convection, and/or radiation, of which conduction is generally preferred for efficient thermal energy transfer. Various connections or links between components may permit thermal conduction, etc., as used herein.
Conduction is the transfer of energy vibrations in matter through other matter, irrespective of relative movement of the molecules. Thermal energy transfer occurs at a higher rate across materials having certain physical properties of high thermal conductivity as compared to across materials or substances of low thermal conductivity. Any selected physical substance has a certain thermal conductivity, ranging from high (e.g., silver or copper) to low (e.g., fabric or wood). Correspondingly, materials of high thermal conductivity are widely used in heatsink applications, and materials of low thermal conductivity are used as thermal insulation. Additionally, thermal conductivity of materials is generally temperature-dependent and conductivity may be a non-linear function of temperature. Generally, conduction occurs across a physical connection between parts, or across areas within one contiguous structure.
Convection is the transfer of thermal energy through motion of the substance itself, such as gas or liquid, where a molecule of a substance moves or circulates from place to place, whereby thermal energy is transported from areas of higher heat to areas of lower heat, or from areas of high density of fluid to areas of low density. The transfer of thermal energy through radiation, however, generally occurs irrespective of a medium for transfer, and is a form of electromagnetic, infrared radiation. Various types of heat shields exist that may act to reduce the thermal transfer efficiency of any of the above-mentioned principles of heat transmission, but such heat shields may be more effective at reducing heat transfer due to radiation and convection, as compared to conduction, depending on the circumstances and configuration. Convection and/or radiation may still occur between distinct parts if a structure has a physical separation or gap between the parts. Conduction can occur through air as well, but in a more limited capacity and efficiency than through physical substances, generally.
In existing SSDs having multiple PCBs in a spaced, parallel arrangement, either nothing or a single internal mechanical or thermal barrier with or without thermal interface material (TIM) separates the PCBs. Some examples of TIM include thermal adhesive and thermal grease. Each PCB is roughly planar and flat, and may have components located on one or both sides of each PCB, such as a top and/or bottom of the PCB. In one example SSD, two PCBs are stacked with each PCB having an inner (on an inward-facing side of a particular PCB) and an outer side (on an outward-facing side of a particular PCB), with the two respective inner sides facing each other, and the two outer sides facing the drive housing (e.g., away from each other as respective outer surfaces). The components on the inner sides' surfaces (inner surfaces) generally have more difficulty (e.g., take more time for) dispersing and dissipating localized heat resulting from greater density of heat-producing components. For example, some components on the inner surfaces of the example two PCBs typically do not have a direct thermal path or conduit to the (e.g., air-cooled by convection) exterior of the drive, and may transfer heat to other adjacent components through which the heat reaches the exterior of the drive housing, before being carried away to a surrounding fluid environment, such as air. Other components may be located adjacent to or nearer to the exterior of the drive housing, as known. Adjacent components, as used herein, may be two or more components in thermal communication or otherwise in close or direct proximity with each other.
It is known that for various substances, thermal energy “seeks” equilibrium and transfers (in the form of heat) from areas of high thermal energy concentration to proximate areas of low concentration, with heat taking a path of least thermal resistance to reduce and equalize stored thermal energy at any particular location. Various thermal paths may exist in a PCB-based, SSD structure, such as heat conductance through a tangible material, convection through a gas, or radiation through electromagnetic radiation, as outlined above. Typically, conduction is substantially more efficient (e.g., more Joules transferred per unit time, such as Watts) and direct in transferring thermal energy than convection, although a rate of thermal conduction varies by substance, as described, above.
An SSD, once assembled, is typically composed of one or more PCBs enclosed in a drive housing having an exterior that is generally exposed to a fluid, such as air, and thus the exterior tends to be a cooler area of the SSD as compared to the internal, more removed from the exterior, components and areas. A drive housing may act as a heat sink, with the exterior emitting thermal energy in the form of radiation and/or convection. A typical heat sink is a passive device configured to absorb (and emit to a fluid, such as air) unwanted or excessive heat, generally from a nearby components attached thereto. In order for the thermal energy of internal SSD components to thermally be cooled by the internal heat reaching the drive housing, it is generally desirable to utilize conduction of heat where possible, using thermal paths from hot (generally internal) areas to cooler (generally external) areas, such as those receptive to air cooling. Additionally, a drive housing may contain various PCBs, with individual, separate heat profiles, that produce relatively more heat than another PCB at a particular moment in time. The heat production of one PCB may disadvantageously thermally affect another PCB, which may be more or less thermally-sensitive, or may, for other reasons, be adversely affected by heat produced by another PCB.
Some components in PCB-based devices may be more thermally sensitive, where, for example, performance thereof is adversely affected by heat above a certain temperature threshold. Alternatively, thermally-sensitive components may have progressively reduced performance as a temperature goes higher or lower than an ideal operating temperature. Examples of thermally-sensitive components that may be located on a PCB or in an SSD may include aluminum, film, polymer, and tantalum ceramic capacitors. On the other hand, some components may be less thermally-sensitive, and may operate under higher temperatures without severe or adverse effects on performance. Also thermally-sensitive may be inductors or transformers with wire coils, crystals, and plastic-based components. Non-solid-state relays and light-emitting diodes (LEDs) are additional examples of potentially thermally-sensitive components. Various thermally-sensitive components, such as electrolytic capacitors may be capable of surviving up to a threshold of, e.g., 120° Celsius, but may suffer drawbacks above that threshold. Various components may react differently to different amounts of heat, as known in the art.
Partially to separate or insulate the heat profiles of the respective PCBs, a typical drive may contain thermal and/or mechanical features, such as a metal or an insulating thermal barrier located between the respective PCBs. The barrier, particularly if not in a path of direct conductance, may at least partially isolate the respective thermal profiles of the PCBs, although the barrier may spread heat to the opposite PCBs more than desired. Therefore, there is a desire to find a more efficient structure for separating multiple thermal profiles of SSD PCBs, while efficiently dispersing heat from higher-heat areas, to relatively lower heat areas, such as a drive housing of an SSD.